AC/DC conversion starting from a three-phase line supply current employs rectifier bridges; in theory, a single bridge with two times three diodes would suffice for rectifying three-phase current into DC current, but in practice, the use of a single bridge powered by the three-phase supply produces a DC current with too large a residual oscillation (ripple), which is unacceptable for many applications. Moreover, the rectification causes a re-injection of currents back into the supply, these currents having harmonics of the frequency of the AC supply current. This re-injection of harmonics is unacceptable if it is too large.
In order to reduce the residual ripple on the DC current and the harmonics injected back into the supply, increasing the number of phases in the supply current and the number of rectifier bridges has already been proposed. Thus, the three-phase system, whose three phases are separated by 120°, may typically be transformed into a system with nine phases separated by 40° which can be considered as a system of three three-phase supplies separated from one another by 40°. Three bridges with six diodes are used, each bridge being powered by one of these supplies. These AC/DC converters with eighteen diodes are also called 18-pulse converters. The residual ripple becomes small, as do the re-injected harmonics. The nine phases are generated using transformers. Autotransformers can be used in order to reduce the weight and dimensions, if there is no constraint on the isolation between the potentials on the line supply side and the potentials on the application side.
The U.S. Pat. No. 5,124,904 describes an 18-pulse converter. The DC voltage obtained from this nine-phase system is higher than that which would be obtained from three phases for various reasons including the fact that the residual ripple is smaller and the DC voltage depends on the mean value of the residual ripple. For reasons of equipment compatibility for example (imposed three-phase voltage, DC voltage of imposed use), this modification of DC voltage level may be undesirable when the rectification using 6 diodes is replaced with an 18-diode rectification. In order to avoid ending up with a higher DC voltage than that which would be produced by a simple three-phase rectification (for the same value of three-phase supply voltage), additional means for reducing the voltage must be provided in the autotransformer. In the U.S. Pat. No. 5,124,904, one embodiment provides these means in the form of additional windings which increase the complexity and the weight, together with the leakage reactance ratio.
The U.S. Pat. No. 5,619,407 proposes a different solution for reducing the DC voltage delivered at the output of the rectifier bridges. This solution does not use additional windings, but it is still unsatisfactory since it results in a non-symmetrical autotransformer structure; this lack of symmetry leads to harmonic distortion and therefore too great a re-injection of harmonics back into the line supply; this distortion is more significant the greater the percentage of reduction in voltage (percentage with respect to the DC voltage that would be delivered by the simple three-phase rectification).
Moreover, the systems described hereinabove do not provide a solution for increasing the DC voltage with respect to that which would be produced by a simple three-phase rectification with six diodes. In fact, there are cases where it can be desirable to increase the DC voltage rather than reduce it.
There is therefore a need for an improved autotransformer which converts a three-phase power supply into a system with nine phases that allows a desired level of DC voltage to be chosen (higher or lower than that which would be produced by a simple three-phase rectification), while at the same time maintaining a low harmonic distortion, and limiting the weight and dimensions of the autotransformer.